The angiotensin I-converting enzyme inhibitor captopril was administrated to 15 renal transplant patients for a minimum period of 14 weeks after transplantation. The object was to decrease ischaemic damage caused by secondary activation of the renal renin-angiotensin system after rejection and associated vascular damage. Captopril patients had lower s-creatinine values than controls in the immediate phase after rejection. On the other hand, after 14 weeks renal perfusion was better, but glomerular filtration rate lower in the captopril group than in the control group. However, none of the differences were significant. Renal function was examined during captopril administration, which may have affected the results. Final evaluation of this trial requires extended follow-up. Captopril was well tolerated in the low doses used (37.5-75 mg daily) in spite of varying degrees of renal failure and concomitant azathioprine administration. Only one case of mild leukopenia was recorded, which improved immediately after captopril interruption. No other serious side effects were observed. Captopril may be safely administered to renal transplant patients, if monitoring of renal function and possible side effects is carried through.